Light detection device

ABSTRACT

A light detection device has a biasing transistor ( 1 ) arranged to provide a bias current and a reverse biased transistor. The reverse biased transistor has a drain terminal ( 6 ) coupled via a high impedance resistor ( 4 ) to the supply voltage. Incident visible light is detected by a voltage drop at the drain electrode.

FIELD OF THE INVENTION

This invention relates to light detection devices and particularly butnot exclusively to light detection devices for use in tamper detectionapplications.

BACKGROUND OF THE INVENTION

In a semiconductor integrated circuit (IC), such as a banking smartcard,the IC is vulnerable to a breach of security if it falls into the handsof a dishonest person. The IC may be reverse engineered in order toreveal or modify functions and confidential data contained therein. Itis known that such IC's have been decapsulated and have even undergonedepassivation of the upper protective layer.

U.S. Pat. No. 4,952,796 describes a circuit which comprises a currentgenerator delivering current which flows into a reversed biasedtransistor junction. If subject to light, the reverse current in thejunction increases, and the voltage at the junction terminals drops.

A problem with this arrangement is that incident light will generatereverse currents in transistors 11 and 2 of FIG.1, and this may affectthe voltage drop detected at the output. Furthermore, the biasing andcurrent generation functions of the above circuit are more susceptibleto manufacturing process variations.

Also current drain, particularly in a smartcard, should be kept to aminimum, and the reverse current adversely affects the currentconsumption of the device. Lastly, the above circuit takes up muchvaluable semiconductor area, and again this is disadvantageous in asmartcard, where demand for space is at a premium.

This invention seeks to provide a light detection device which mitigatesthe above mentioned disadvantages.

SUMMARY OF THE INVENTION

According to the present invention there is provided a light detectiondevice comprising: a biasing transistor, arranged to provide a biascurrent; a reverse biased transistor having a control electrode arrangedto be reverse biased by the bias current and having a conductingelectrode; and, a resistor coupled between a supply voltage and theconducting electrode of the reverse biased transistor; wherein incidentvisible light is detected by a voltage drop at the conducting electrodeof the reverse biased transistor.

In this way a light detection device is provided which does not generateparasitic reverse currents, and is less susceptible to manufacturingprocess variations.

BRIEF DESCRIPTION OF THE DRAWING(S)

An exemplary embodiment of the invention will now be described withreference to the single figure drawing which shows a preferredembodiment of a light detection device in accordance with the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to the single figure drawing, there is shown a light detectiondevice 10, arranged to be integrated with an IC.

The device 10 comprises a first transistor 1, having a gate terminalcoupled to a supply voltage Vdd, a source terminal coupled to a groundterminal Vss and a drain terminal coupled to the supply voltage Vdd viaa first high impedance resistor 3. The first transistor 1 is thusarranged to provide a bias current to be further described below.

A second transistor 2 of the device 10 has a gate terminal coupled tothe drain terminal of the first transistor 1, a source terminal coupledto the ground terminal Vss and a drain terminal 5 coupled to the supplyvoltage

Vdd via a second high impedance resistor 4, and further coupled to anoutput terminal 6. The high impedance resistor 4 is an undopedpolysilicon resistor.

In operation, the gate terminal of the second transistor 2 is arrangedto be reverse biased by receiving the bias current from the drainterminal of the first transistor 1.

When the drain terminal 5 of the second transistor is subjected toincident visible light, a small reverse current is generated betweendrain 5 and the ground terminal Vss. This current flow lowers thevoltage at drain 5, and this voltage drop is detected by circuitry (notshown) coupled to the output terminal 6.

Since the second high impedance resistor is an undoped polysiliconresistor, extremely small currents can be detected, making the device 10very sensitive to incident light.

No parasitic reverse currents are generated, and during themanufacturing process of the device 10, only one resistivity process isrequired to fabricate the first and the second high impedance resistors3 and 4, and this will not introduce variations which will significantlyaffect the performance of the device 10.

It will be appreciated that alternative embodiments to the one describedabove are possible. For example, the biasing arrangement may vary fromthe precise configuration described above. In addition, the first andsecond resistors could be fabricated from a material other than undopedpolysilicon.

What is claimed is:
 1. A light detection device comprising: a biasingcircuit arranged to provide a bias current; a reverse biased transistorhaving a control electrode arranged to be reverse biased by the biascurrent and having a conducting electrode arranged to receive incidentvisible light; and, a high impedance resistor coupled between a supplyvoltage and the conducting electrode of the reverse biased transistor;wherein incident visible light is detected by a voltage drop at theconducting electrode of the reverse biased transistor.
 2. The device ofclaim 1 wherein the resistor is an undoped polysilicon resistor.
 3. Thedevice of claim 1, or 2 wherein the biasing circuit comprises a biasingtransistor having a conducting electrode coupled to the supply voltagevia a biasing resistor.
 4. A smart-card incorporating the device ofclaim
 1. 5. A light detection device comprising: a reverse biasedtransistor having a control electrode and a conducting electrode, saidconducting electrode being arranged to receive incident light; a biasingcircuit arranged to provide a bias current to the control electrode ofthe reverse biased transistor; and a high impedance resistor coupledbetween a supply voltage and the conducting electrode of the biasedtransistor, said high impedance resistor providing a constantresistance; wherein incident visible light is detected by a voltage dropat the conducting electrode of the reverse biased transistor.